In vitro studies on replication of plasmid Col E1 DNA has been the major subject of investigation. The origin of replication was located at a unique site in the nucleotide sequence that we determined. Ribonucleotide(s) was found to attach to a DNA fragment first formed in the process of plasmid replication. The participation of various bacterial functions for the plasmid replication was studied. The gene that determines the resistance to novobiocin was identified as that for DNA gyrase. BIBLIOGRAPHIC REFERENCES: Sakakibara, Y., Suzuki, K. and Tomizawa, J.: Formation of Catenated Molecules by Replication of Colicin E1 Plasmid DNA in Cell Extracts. J. Mol. Biol. 108: 569-582, 1976. Tomizawa, J., Ohmori, H. and Bird, R.E.: The Origin of Replication of Colicin El Plasmid DNA (cleavage map/nucleotide sequence/polynucleotide kinase/primer). Proc. Natl. Acad. Sci. U.S.A. 74: 1865-1869, 1977.